Borrelia burgdorferi is a tick-borne spirochete and the causative agent of Lyme disease. The focus of this renewal application is the biology of Lyme disease spirochetes within the tick vector. Studies have demonstrated that spirochetes delivered by ticks express different antigens compared to cultured spirochetes. Tick-borne spirochetes are able to better evade host immunity than cultured organisms. In this application we propose to use recently developed genetic tools to identify and study the function of Borrelia genes expressed in the vector. In aim 1 we will focus on two Borrelia plasmids (lp25 and lp28-4) required for tick infection. We will identify specific genes on these plasmids required for infecting ticks. The focus of aim 2 will be two Borrelia outer membrane lipoproteins designated OspA and OspC. We will further refine existing models about the function of these proteins in ticks by using Borrelia mutants that inappropriately express these proteins. Studies of the interactions between Borrelia and ticks are significant because they have the potential to lead to vaccines that block transmission. We have demonstrated that specific antibody entering ticks is able to block the transmission of spirochetes by more than one mechanism. In aim 3 we propose experiments to further dissect how Borrelia specific antibody entering ticks influence the phenotype of spirochetes and their transmission to the host. NARRATIVE Lyme disease is a tick-borne disease of people that is common in the USA and other parts of the world. The goal of this proposal is to study how Lyme disease bacteria are transmitted by ticks. The work is applicable to developing novel vaccines that prevent ticks from acquiring or transmitting Lyme disease bacteria.